Flame-based detection is a common technique used in chromatography (e.g. gas chromatography) to detect analytes of interest (e.g. organic compounds) in an analyte stream. For instance, flame ionization detection (FID) functions by maintaining a flame via the addition of a combustible fuel (e.g. hydrogen) and an oxidant (e.g. air or oxygen) to the detector. An analyte stream (e.g. the eluent from a gas chromatography column) passes through the flame in the flame ionization detector. Compounds that contain a reduced form of carbon (e.g. organic compounds from the analyte stream that contain C—H bonds) are ionized in the flame to produce carbon-based ions and free electrons, while compounds without reduced carbon such as carbon dioxide, nitrogen and noble gases (e.g. helium) do not form free electrons. The newly-generated free electrons are attracted to a positive electrode (e.g. anode) while the carbon-based ions are attracted to a negative electrode (e.g. cathode) above the flame. As the ions and electrons reach their respective electrodes, an electric current is established. The amount of current flow is thus proportional to the number of carbon atoms entering the flame ionization detector. Accordingly, flame ionization detectors are very selective for, and can accurately measure the presence of, analytes that contain a reduced form of carbon (e.g. organic compounds).
In general, there are a number of different mobile phase fluids used in chromatography. Various chromatographic systems can use different mobile phase fluids depending on the nature of the separation to be carried out. For instance, any one of liquid acetonitrile, helium gas, or carbon dioxide can serve as a mobile phase in a chromatographic context. In addition, while operating with, for instance, a carbon dioxide mobile phase, modifiers can be added to the mobile phase to change the mobile phase net polarity and separation characteristics.
Although mobile phase polar modifiers such as methanol can serve to enhance the separation of analytes in a given sample of interest, many modifiers (e.g. methanol) contain a reduced form of carbon and therefore respond in a flame-based detector (e.g. a flame ionization detector). Because a polar modifier can be present in an amount much greater than the analyte of interest, the response from the flame-based detector to the polar modifier can overwhelm the response from the flame-based detector to the analyte of interest.